TY - JOUR T1 - Determinants of base-pair substitution patterns revealed by whole-genome sequencing of DNA mismatch repair defective <em>Escherichia coli</em> JF - bioRxiv DO - 10.1101/346874 SP - 346874 AU - Patricia L. Foster AU - Brittany A. Niccum AU - Ellen Popodi AU - Jesse P. Townes AU - Heewook Lee AU - Wazim MohammedIsmail AU - Haixu Tang Y1 - 2018/01/01 UR - http://biorxiv.org/content/early/2018/06/14/346874.abstract N2 - Mismatch repair (MMR) is a major contributor to replication fidelity, but its impact varies with sequence context and the nature of the mismatch. Mutation accumulation experiments followed by whole-genome sequencing of MMR-defective E. coli strains yielded ≈30,000 base-pair substitutions, revealing mutational patterns across the entire chromosome. The base-pair substitution spectrum was dominated by A:T &gt; G:C transitions, which occurred predominantly at the center base of 5′NAC3′+5′GTN3′ triplets. Surprisingly, growth on minimal medium or at low temperature attenuated these mutations. Mononucleotide runs were also hotspots for base-pair substitutions, and the rate at which these occurred increased with run length. Comparison with ≈2000 base-pair substitutions accumulated in MMR-proficient strains revealed that both kinds of hotspots appeared in the wild-type spectrum and so are likely to be sites of frequent replication errors. In MMR-defective strains transitions were strand biased, occurring twice as often when A and C rather than T and G were on the lagging-strand template. Loss of nucleotide diphosphate kinase increases the cellular concentration of dCTP, which resulted in increased rates of mutations due to misinsertion of C opposite A and T. In an mmr ndk double mutant strain, these mutations were more frequent when the template A and T were on the leading strand, suggesting that lagging-strand synthesis was more error-prone or less well corrected by proofreading than was leading strand synthesis. ER -